In the past, there has been great interest in determining the load carried on trucks for hire. Loads carried by trucks are often supported by pallets, because pallets provide a stable platform on which to position and transport goods between trucks, using a hydraulic lifting device. Typically, prior art weight measurement devices were concerned with measuring the change in the gross weight of a truck to determine whether it complied with regulations relating to the loading of commercial motor vehicles and/or determining the amount to charge a customer for transportation of a particular load. This does not allow determining the weight of individual pallets of goods, before loading the pallets onto the truck, which may result in overloading of the same. To prevent overloading of the truck, the pallets of goods may be individually transported to a scale where they may be weighed. A problem with this procedure is that it is time consuming, resulting in increased transportation costs for a given load. An obvious solution to this problem would be to provide a scale for each truck receiving a load, thereby providing dynamic weighing of the total load of the truck as pallets of goods are placed thereon. This would require having a separate scale for each truck receiving pallets of goods, or creating a queue of trucks for each scale present so that each may, in turn, be placed on the scale during loading. Both of these solutions result in the same drawbacks as individually weighing pallets of goods. Alternately, the pallets of goods may be weighed by the hydraulic lifting device used to move the pallets to and from the trucks.
Prior art attempts have been made for dynamically weighing loads moved by hydraulic lifts. Weigh-Tronix, Inc. describes, in a sales brochure, a device for dynamically weighing loads supported by a forklift. The Weigh-Tronix device includes a large frame fitting between a carriage and a pair of forks. The frame is mounted parallel to the carriage and includes upper and lower spaced apart plates, extending parallel to, and contiguous with, the carriage along a first direction. The plates are joined by flexible members, extending perpendicular to the first direction, parallel to the carriage. The forks are "L" shaped with a first portion mounted to the upper plate and extending parallel thereto, terminating in a lower portion, proximate to the lower plate. The lower portion extends orthogonally from the lower plate. Deflection of the flexible members is proportional to a weight disposed on the forks. A drawback with the Weigh-Tronix device is that its weight is excessive, making is difficult to install and costly to manufacture, as well as to transport to an end user. In addition, the accuracy of the Weigh-Tronix device degrades over time due to fatiguing of the flexible members, which, if overloaded, will permanently deform, making the device susceptible to premature failure.
To overcome the excessive weight of the frame in the Weigh-Tronix device, prior art devices have placed a weight transducer in the hydraulic lift circuit. In this manner, a portion of the hydraulic fluid is transmitted along a bypass from the main lift circuit, where the transducer is positioned, to measure the pressure of the fluid in the bypass. U.S. Pat. Nos. 5,287,885; 5,195,418; and 5,139,101 to Smith each discloses such a bypass system. Specifically, a motion control system for hydraulically operated lifting devices is shown including, in pertinent part, a two-way valve having a normally open valve in one chamber and a normally closed valve in another chamber. The normally open valve may be closed to re-direct flow of hydraulic fluid from a main valve under pressure from a hydraulic pump to a bypass chamber having a flow control valve. The normally closed valve may be opened to direct flow from a lift circuit of a second flow control valve to pass the hydraulic fluid back to a hydraulic fluid reservoir tank. The operation of opening and closing the aforementioned valves makes different flow paths for the hydraulic fluid so as to more closely control the ascent and descent of a load being displaced.
U.S. Pat. Nos. 5,065,829; 5,065,828; and 5,064,008 to Smith each discloses a hydraulic control system for weighing including, in pertinent part, a bypass control system that slowly releases hydraulic fluid from a lift circuit to direct it back to a reservoir in a controlled manner. This allows a load to be smoothly moved from a reference position to a weighing position, reducing the degradation of the measurement's accuracy due to vibration and shaking.
U.S. Pat. No. 5,327,347 to Hagenbuch discloses a method and apparatus for accurately measuring haulage parameters including, in pertinent part, a processing unit in electrical communication to receive signals from a plurality of pressure transducers. Each of the pressure transducers transmits signals to the processing unit corresponding to the weight of load present thereon, establishing a historical data base reflecting the vehicle operating conditions. To obtain an accurate weight measurement, the processing unit reads the pressure measurement from each transducer sixteen times and averages the sixteen measurements together. The remaining transducers are each, in turn, measured sixteen times with the readings averaged. After all the readings from each transducer is averaged, the averaged readings are then summed and averaged, providing a net average measurement of the pressure from all of the transducers. The net average measurement corresponds to the weight of the load on the vehicle.
U.S. Pat. No. 5,210,706 to Nishiyama discloses a device for measuring weight including a plurality of load cells. Signals transmitted by the load cells represent pressure changes which correspond to the weight of a load to which the load cells are subjected. The signals are sent to analog-to-digital converters where they are digitized. Low frequency noise components are reduced by digital filters in electrical communication with the analog-to-digital converters. The reduced noise signals are then summed to provide weight data.
U.S. Pat. No. 5,243,512 to Putman et al. discloses a method and apparatus for minimizing vibration including, in pertinent part, a plurality of accelerometers and actuators, with the accelerometers attached to a system of interest producing vibrations. The accelerometers detect vibration and produce electrical signals corresponding thereto. A processor is coupled to receive the electrical signals and measures the vibrational characteristics of the system using one of three algorithms. The processor then directs the actuators attached to the system to provide a compensating force, thereby minimizing the vibrational characteristics of the system.
A problem with the aforementioned devices is that the accuracy of a weight measurement is degraded due to various mechanical disturbances such as shaking and vibration, as well as pressure surges in the hydraulic fluid. These environmental disturbances substantially degrade the accuracy of the weight measurement.
What is needed is a hydraulic lifting device capable of obtaining a high precision weight measurement, dynamically, without degradation of the measurement's accuracy by vibration, shaking or other mechanical disturbances.